Backport from ELPA repository.

* tex-site.el (TeX-modes-set): Use advice-add if available.
* font-latex.el (font-latex-make-sectioning-faces): Don't rely on
dynamic scoping for `num'.
(font-latex-make-built-in-keywords): Don't use `eval' needlessly.
(font-latex-doctex-syntactic-keywords): Declare before first use.
(font-latex-match-command-with-arguments):
Stay away from `add-to-list' on let-bound variables.
(font-latex-match-command-in-braces): Remove unused var `end'.